Boyfriend asked me to marry him and I'm not ready, advice?

My boyfriend who I love very much asked me to marry him a couple of days ago.We've been together 18 months and I couldn't be more in love but I'm only 22 and I'm not even sure I believe in marriage.It doesn't mean I don't want to be with him or love him any less but I had to turn him down and how he is really upset and won't speak to me at all.What can I do?

he doesn't have to think your TRYING to hurt him -yo be hurt. for me its worse when its genuine. like saying I don't want to be with u, means less if your only trying to hurt someone. saying it because its true, hurts way more, because its honest.

he needs time. its not your fault. but he probably feels ashamed. I would feel very uncomfortable & a bit degraded. not because of the guy not wanting me,per se.

but because I was ready to go the distance &it was not reciprocated. its embarrassing,